The present invention relates generally to oscillating wave engines and thermoacoustic engines, including Stirling engines and thermoacoustic-Stirling hybrids.
A variety of oscillating thermodynamic engines and refrigerators have been developed, including Stirling engines and refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Much of the evolution of this entire family of oscillating thermodynamic technologies has been driven by the search for higher efficiencies, greater reliabilities, and lower fabrication costs.
Some combinations of one or more thermoacoustic engines and one or more thermoacoustic refrigerators or orifice pulse tube refrigerators, such as the thermoacoustic-Stirling hybrid engine driving three orifice pulse tube refrigerators shown in FIG. 1, have provided heat-driven refrigeration with no moving parts whatsoever. Such systems with no moving parts can yield the greatest reliability and lowest fabrication costs. As used herein, thermoacoustic engines mean both standing-wave thermoacoustic engines, in which stacks are used; thermoacoustic-Stirling hybrid engines, in which regenerators are used; and Stirling engines, in which regenerators are used.
FIG. 1 schematically shows one such prior art combined system 10. This combined system comprises a chain of energy-conversion hardware: a natural-gas-fired burner 12, which provides heat to a thermoacoustic-Stirling hybrid engine 14, which in turn provides acoustic power to an orifice pulse-tube refrigerator 16, which in turn cools and liquefies a purified natural gas stream. The conversion of heat to acoustic power occurs in regenerator 18 of engine 14, which is a solid matrix smoothly spanning the temperature difference between hot heat exchanger 22 and main ambient heat exchanger 24 of the engine and containing small pores through which the gas oscillates. The conversion of acoustic power to refrigeration takes place similarly in regenerator 26 spanning a temperature gradient between ambient heat exchanger 28 and cold heat exchanger 32.
Proper design of the resonator and total thermoacoustic system shown in FIG. 1 ensures that the system oscillates spontaneously at a desired frequency, called the resonance frequency, when it is operating. Acoustic energy stored in the resonance, comprising kinetic energy of oscillating motion and compressional energy of oscillating pressure, acts like a flywheel so that acoustic power production in the engine and acoustic power consumption in the refrigerators can take place at arbitrarily different temporal phasing within each cycle of the wave. Proper design of the acoustic network in which the engine""s regenerator and heat exchangers are imbedded causes the gas in the pores of the engine""s regenerator 18 to move toward hot heat exchanger 22 while the pressure is high and toward main ambient heat exchanger 24 while the pressure is low, these oscillations occurring at the resonance frequency.
Thus, the oscillating thermal expansion and contraction of the gas in regenerator 18, attending this oscillating motion along the steep temperature gradient in the pores, is temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low. This expansion and contraction, properly phased with the oscillating pressure, is the thermodynamic work that produces acoustic power in engine 14, maintaining the oscillation against consumption of acoustic power by the loads. The load comprises, e.g., the refrigerator 16 and also dissipative effects throughout the system.
The discussion above relies strongly on sufficient steepness of the temperature gradient in regenerator 18 along the direction of the oscillating motion, but is only weakly dependent on the amplitude of the oscillation. With ambient temperature fixed, the steepness of the temperature gradient is controlled by the temperature of hot heat exchanger 22, henceforth called the hot temperature. Indeed, standing-wave thermoacoustic engines and thermoacoustic-Stirling hybrid engines can operate stably over a very broad range of oscillation amplitudes once the hot temperature exceeds a certain temperature, called the threshold temperature herein, with higher amplitudes associated with higher hot temperatures.
The threshold temperature depends on many details of the entire thermoacoustic system, including, in FIG. 1, the load provided by refrigerator 16. A greater load (e.g. an additional refrigerator that might be connected in parallel) would cause a higher threshold temperature. Low oscillation amplitudes are encountered when the hot temperature is only slightly above the threshold temperature, while the higher amplitudes are achieved when the hot temperature is significantly hotter than the threshold temperature. High amplitude is desirable in order to achieve the highest acoustic power, and thermoacoustic systems are typically designed for routine operation at a high amplitude, called herein the design operating amplitude. In stable operation at any amplitude, a balance exists between the acoustic power produced by the engine and the acoustic power consumed by loads such as refrigerators and dissipative effects throughout the system.
Typically, to start such an engine, beginning from a state in which all parts of the engine are at ambient temperature, burner 12 is ignited and begins producing heat. At first, the heat from burner 12 simply warms the massive parts of hot heat exchanger 22, burner 12 itself, and any hardware (not shown) associated with burner 12, such as a counterflow recuperator that might pre-heat the fresh air delivered to burner 12 by capturing waste heat from the exhaust downstream of burner 12 and hot heat exchanger 22. Hence, at first the temperatures of these parts of the system simply increase with time, and no thermoacoustic oscillations occur. The rate of temperature increase of these temperatures depends on the output from burner 12 and the heat capacity of these parts of the system.
When the hot temperature finally reaches the threshold temperature, the thermoacoustic oscillations begin spontaneously at the resonance frequency, typically at an amplitude that is much smaller than the design operating amplitude. Increases in burner 12 power then increase the hot temperature and the amplitude of the oscillations, with most of the additional burner power going into the thermoacoustic processes. Eventually the design operating amplitude is reached, and the oscillation amplitude stabilizes with burner 12 supplying a fixed amount of heat.
Typically, to effect a complete shutdown of such an engine, beginning from a state in which it is oscillating at high amplitude, such as its design operating amplitude, burner 12 power is reduced or eliminated, and the temperature of hot heat exchanger 22 begins to fall due to consumption of heat by the thermoacoustic processes in the engine and due to heat leak from hot heat exchanger 22 to ambient. As the hot temperature falls, the amplitude of the oscillations decreases, and hence the rate of fall of temperature may decrease. Eventually, the hot temperature falls below the threshold temperature, and the oscillations cease. Further decrease in the hot temperature toward ambient then occurs, usually caused by heat leak from hot heat exchanger 22 to ambient temperatures, but sometimes accelerated by circulation of air or water.
Some hysteresis between turn-on threshold temperature and shutdown threshold temperature may occur, but this does not affect the operation of the present invention as described herein.
The entire startup procedure can be as long as many hours, depending on the heat capacity of the parts that must be heated and on other factors such as the need to avoid thermal-shock damage, i.e. overstressing parts by causing excessively steep temperature gradients. Both the time to safely heat the hot parts from ambient temperature to the threshold temperature and the time to safely heat them from the threshold temperature to the design operating temperature can be long. Similarly, the shutdown procedure described above can take a long time, with oscillations diminishing in amplitude during a period ranging from minutes to hours, depending on the specific hardware, and then cooling from the threshold temperature to ambient taking additional hours.
There are many circumstances in which the state of the art described above is unsatisfactory. For example:
1. A failure of burner 12 causes shutdown to proceed at a rate determined by the thermoacoustic phenomena which remove stored heat from the heat capacity of hot heat exchanger 22 and nearby hot parts. That rate might be too rapid and cause thermal-shock damage to the hot parts. Thus, it would be desirable to enable slower cooling even while the burner is inoperative.
2. A failure in another part of a complex facility in which thermoacoustic system 10 is imbedded might call for a rapid shutdown of thermoacoustic system 10. For example, failure of a methane pump that is supplying methane to refrigerators 16 might call for immediate shutdown of the thermoacoustic oscillations before the unloaded refrigerators cool below the freezing point of methane and create a plug of frozen methane in cold heat exchangers 32.
3. A temporary, rapid shutdown and restart of thermoacoustic system 10 might be desired, such as while quick repairs to a non-thermoacoustic portion of the facility are made.
Accordingly, there is a need to provide to thermoacoustic engines a means for rapid stopping and starting of the thermoacoustic oscillations, preferably with hardware that is simple, reliable, and cheap.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and lo broadly described herein, the present invention includes a thermoacoustic engine-driven system with a hot heat exchanger, a regenerator or stack, and an ambient heat exchanger, with a side branch load for rapid stopping and starting, the side branch load being attached to a location in the thermoacoustic system having a nonzero oscillating pressure and comprising a valve, a flow resistor, and a tank connected in series. The system is rapidly stopped by simply opening the valve and started by simply closing the valve.